Advanced processes, 90 nm and below, use extremely thin and sensitive layers. Such layers can be affected by a very small amount of contamination (e.g., less than a monolayer). When a thin contamination layer covers such a sensitive process layer, the process yield can be negatively affected. The contamination level that can cause yield problems is so small and various prior art techniques such as SIMS and RGS are not suitable to measured this layer, due to the contamination they add and due to their limited sensitivity.
An example of an extremely sensitive layers are the Copper Barrier layer and the Copper Seed layer. The thickness of these layers can range between few nanometers to less than a nanometer. Even a sub-monolayer of contamination materials (for example organic layers, hydrocarbons, fluorocarbons, etc.) deposited on these layers can damage overall yield. Such contamination layers are formed in various process and process monitoring tools, including in Scanning Electron Microscopes. The challenge is to reduce the level of contamination, so that yield will not be affected.
One prior art method for monitoring the cleanliness of wafers is the contact angle measurement. Contact angle measurement describes a shape of a liquid droplet that rests on the wafer. The shape of the droplet is represented by an angle between the surface of the wafer and an imaginary tangent line from the droplet to the solid surface.
A conventional contact angle measurement device includes a stage that holds the wafer, a syringe to apply a droplet of liquid, an illuminating light source and a camera that detects the shape of the droplet. The camera is connected to an image processor that can determine the contact angle.
Wafers are processed during a highly complex manufacturing process. During the manufacturing process the wafers travel between multiple manufacturing tools and quality control tools such as wafer inspection tools, defect review tools, metrology tools and the like. These tools are often very large and the wafer travels relatively long distances between one tool to the other.
Contamination and especially very thin (even monolayer) films are formed very quickly. For example, copper layers must be transferred in a short time between one tool to another.
In a typical manufacturing facility multiple wafers are manufactured, while the timing of certain operations relating to a single wafer can slightly differ from tool to tool, and from operator to operator.